Clinical and Laboratory Studies are conducted to determine etiology (infection, immunity and/or genetics) for chronic diseases of the central nervous system. Current studies include Multiple Sclerosis, Progressive Multifocal Leukoencephalopathy, Subacute Sclerosing Panencephalitis, Myasthenia Gravis, Amyotrophic Lateral Sclerosis and Parkinson's Disease. Combined clinical data, genetic information, HLA and MLC typing, virus serology and virus isolation studies are obtained in these investigations. New prophylactic and therapeutic materials are studied for the prevention and control of these diseases in volunteers and patients. Current studies include evaluation of measles vaccines for immunogenicity in relation to Subacute Sclerosing Panencephalitis and cellular and humoral immunity to individuals with Progressive Multifocal Leukoencephalopathy.